P0670
PowertrainGlow Plug Module Control Circuit
The glow plug control module is the box that decides when your glow plugs get fed power to pre-heat the combustion chambers before a cold start, and it talks back to the ECU about what it's doing. P0670 means the ECU has lost confidence in the electrical circuit running that module. On a diesel that usually shows up first thing on a frosty morning, when the engine cranks and cranks before it catches.
ⓘ Information only. This page provides general educational information about fault code P0670. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0670 mean?
P0670 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Glow Plug Module Control Circuit.
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Hard starting on cold mornings, the engine cranks longer than usual before it fires
- • Lumpy, hunting idle for the first 30 seconds to a minute until everything warms through
- • Engine warning light on, sometimes alongside a separate glow plug coil light staying lit
- • White or grey smoke from the exhaust on a cold start as fuel goes in unburnt
- • A drop in fuel economy that creeps in over short trips
- • On some cars the module fault drops it into a reduced glow strategy, so warm starts are fine but cold ones are miserable
Possible causes
Causes commonly associated with P0670, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. The glow plug control module itself has failed internally, common on VAG diesels like the 1.6 and 2.0 TDI and on some BMW and Mercedes oil-burners where the module lives in a damp spot
- 2. Corroded or chafed wiring in the control circuit, often where the loom runs near the engine and picks up heat and road salt
- 3. A blown fuse or fusible link in the glow plug supply, worth ruling out before you condemn anything pricey
- 4. Poor earth or a corroded multiplug at the module connector, dirty pins throw the readings off
- 5. One or more failed glow plugs dragging the circuit out of spec and confusing the module
- 6. ECU fault, which is rare but does happen if the driver stage inside has cooked
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Read the live data and freeze frame, then check what other codes are sitting alongside P0670. A flurry of P0671 to P0674 points at the plugs, a lone P0670 points at the module or its wiring
- 2. Check the glow plug fuse and the supply feed to the module before anything else, it's a five minute job and saves stripping a loom for nothing
- 3. Inspect the module connector and earth point for green corrosion or a loose pin. Unplug it, look at the terminals, give them a clean if they're crusty
- 4. Back-probe the module for battery voltage on the supply side and a solid earth with the ignition on. No feed means you're chasing wiring, not the module
- 5. Test each glow plug at the tip with a multimeter, you're looking for roughly 0.5 to 2 ohms. Anything reading open is dead and needs replacing
- 6. If supply, earth, wiring and plugs all check out, the module is the likely fault. Clear the codes and recheck on the next cold start to confirm
Common questions about P0670
Can I keep driving or do I need to sort it straight away? +
Once it's started and warm, the car will usually drive fine, so it's not a roadside emergency. The catch is the cold starts. Leave it through a UK winter and you risk the morning it won't fire at all, plus all that unburnt fuel washes the bores and isn't kind to the engine. Get it looked at before the cold weather really sets in rather than after it leaves you stranded on the drive.
Is it the control module gone bad, or just the wiring and connections? +
Either, and you can't tell which from the code alone. Plenty of P0670s turn out to be a corroded earth, a tired connector or a blown fuse, which is a cheap fix. The module itself does fail, especially the ones mounted low down where they cop water and salt, but it costs a lot more so prove the wiring is clean first. Check the easy stuff before you buy a £150 to £300 module you might not have needed.
How long does the repair actually take? +
A wiring or connector repair is often an hour or two of labour once the fault is pinned down. A fuse is minutes. Swapping the control module is usually a half-day job depending on where the manufacturer has buried it, and on a VAG or BMW diesel you have to add time for coding the new unit so the ECU recognises it. Most independent garages will have it back to you the same day.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →